CN103686533B - System and method for transmitting a radio frequency signal through a speaker coil - Google Patents

Abstract

Systems and methods for transmitting radio frequency signals through speaker coils. According to an embodiment, a system has an audio amplifier configured to be coupled to a speaker coil port through a parallel resonant circuit, and a radio frequency (RF) amplifier configured to transmit RF signals at a first RF transmit frequency. The speaker coil port is configured to couple to a speaker coil, and the parallel resonant circuit has a resonant frequency at about a first RF transmit frequency.

Description

Systems and methods for transmitting radio frequency signals through speaker coils

technical field

The present invention relates generally to semiconductor circuits and methods, and more particularly to systems and methods for transmitting radio frequency signals through speaker coils.

Background technique

Because of its electrical properties similar to those of ferrite rod antennas, speaker coils can be used as antennas for radio frequency (RF) transmitting devices, as well as electromechanical drivers for loudspeakers at audio frequencies. In cost-sensitive portable devices such as mobile phones, digital audio devices, headphones and hearing aids, RF functionality as well as acoustic functionality can be added without taking up a lot of additional physical space. For example, adding RF functionality to a speaker coil within a mobile phone could enable near-field communication for financial transactions or for exchanging data between users. For hearing aids, the addition of RF functionality may allow hearing aids to be controlled or programmed remotely, and may enable binaural hearing aid pairs to exchange data to improve the directionality of audio signals using audio signal processing algorithms.

In some systems, a combined acoustic and RF transmission system can be achieved by coupling audio and RF power amplifiers in parallel with the speaker coils. While the RF power amplifier may use an AC coupling capacitor to prevent interference with the audio signal, this AC coupling relative to the audio amplifier may present a capacitive impedance that reflects the RF signal generated by the RF power amplifier. In some systems, such reflections may be handled by using lossy high-pass and low-pass filter networks that may cause loss and attenuation to the audio signal. In some cases, these losses are due to losses in the filter inductors for RF frequencies in the range of 100 kHz up to 30 MHz. Inductors with very small form factors, such as those commonly used in portable devices, can be particularly lossy. In other systems, the RF amplifier and the acoustic amplifier may be coupled to the speaker coil through mechanical relays.

Contents of the invention

According to an embodiment, a system has an audio amplifier configured to be coupled to a speaker coil port through a parallel resonant circuit, and an RF amplifier configured to transmit an RF signal at a first RF transmission frequency. The speaker coil port is configured to couple to a speaker coil, and the parallel resonant circuit has a resonant frequency at about the first RF transmit frequency.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, subjects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

Description of drawings

For a more complete understanding of the invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

Figure 1 shows a schematic diagram of an embodiment system where an RF amplifier is coupled to a loudspeaker through a magnetic transformer;

Figure 2 shows a schematic diagram of an embodiment system in which an RF amplifier is coupled to a loudspeaker through a balun transformer;

Figure 3 shows a schematic diagram of an embodiment system in which an audio amplifier is coupled to parallel loudspeakers through a resonant circuit with a bypass switch;

Figure 4 shows a schematic diagram of an embodiment system in which an audio amplifier is coupled to a loudspeaker through a tunable parallel resonant circuit; and

Figure 5 shows a schematic diagram of another embodiment system in which an audio amplifier is coupled to a loudspeaker through a tunable parallel resonant circuit.

Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate relevant aspects of the preferred embodiments and are not necessarily drawn to scale. Letters denoting variations of the same structure, material, or process step may follow the figure number in order to more clearly illustrate a particular embodiment.

detailed description

The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific situations. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.

The invention will be described with respect to an embodiment in the specific case, namely an RF transmission system using a loudspeaker coil. Embodiments of the present invention are not limited to systems using speaker coils, but may be applied to other circuit types. Examples of other circuits include, but are not limited to, wireline and wireless communication circuits, and circuits using multiple transmit amplifiers and/or signal generators.

In an embodiment, the audio amplifier and RF amplifier are coupled to the speaker coil using a low loss matching network which may include one or more CMOS switches, inductors and capacitors. In some embodiments, the matching network may be adjustable and/or adaptable to compensate for changing RF environments in the vicinity of the speaker coil and/or to compensate for variations in manufacturing. Accordingly, some embodiments may utilize a tunable filter implemented using an inductor coupled to a switchable capacitor bank with switching capability by one or more high-power transistors, such as RF CMOS transistors or other types of transistors. of one or more capacitors. In some cases, high efficiency performance is maintained even if the power transistor has poor RF isolation or even if the inductor has a relatively large series resistance compared to the resistance of the high power transistor or the sum capacitor.

FIG. 1 illustrates an embodiment acoustic and RF transmission system 100 that includes an audio amplifier 102 coupled to a speaker coil 106 through a secondary winding of a transformer 110 . RF amplifier 104 is further coupled to speaker coil 106 through transformer 110 such that the primary winding of transformer 110 is coupled to RF amplifier 104 . In an embodiment, the transformer 110 may be implemented as a current-type transformer having a low impedance sized for the sum of both the RF and audio signal currents. The turns ratio of the transformer may be sized to provide maximum power matching between speaker coil 106 and RF power amplifier 104 . Because transformer 110 provides AC isolation, additional decoupling for the RF power amplifier is required.

For RF signals, capacitors 114 and 116 provide a low impedance path to ground. In some embodiments, capacitors 114 and 116 may be implemented using existing parasitic capacitance of devices coupled to transformer 110 and/or additional capacitive components, if necessary, to properly bypass the RF signal generated by RF amplifier 104 . Similarly, capacitors 108 and 112 represent the parasitic capacitances of the primary and secondary windings, respectively, of voltage transformer 110 plus additional capacitive components that can be used to tune the parallel resonant frequency of the transformer to approximately 104 to generate the transmit frequency of the RF signal. By operating transformer 110 in parallel resonant mode, smaller transformers can be used compared to those used for non-resonant operation. In some embodiments, transformer 110, RF amplifier 104, and audio amplifier 102 may be implemented on the same integrated circuit. Alternatively, these components or a combination of these components may be implemented using multiple integrated circuits and/or board level components that are partitioned in different ways.

In an embodiment, the transformer 110 may be configured to have a coupling coefficient of 0.98 or greater at low frequencies. Alternatively, other coupling coefficients may be used depending on the particular system and its specifications. In some embodiments, transformer 110 may have a 1:1 turns ratio between the primary and secondary sides of transformer 104 . In some embodiments, transformer 110 may be implemented using a ferrite core transformer (eg, using from about 5 windings to about 7 windings per side). Alternatively, other turns ratios and/or other transformer types may be used depending on the particular embodiment and its specifications.

RF amplifier 104 may be configured to transmit RF signals at frequencies between about 100 kHz and about 30 MHz, however, frequencies outside this range may be used. For example, in an embodiment where a 13 MHz transmit frequency is used, the parallel resonance due to the inductance of transformer 110 and the combination of capacitors 108 and 112 may also be tuned to an RF frequency of 13 MHz. In some embodiments, RF amplifier 104 may be implemented as a single-ended amplifier providing, for example, 100 mW or less of power to speaker coil 106 . RF amplifier 104 may alternatively provide higher power than 100 mW and/or may be implemented as a differential amplifier.

In an embodiment, the audio amplifier 102 may be implemented using a class D amplifier having, for example, a bridge-type output stage driven by a pulse width modulated output stage (not shown). In some embodiments, a low pass filter 101 may be coupled between the audio amplifier 102 and the transformer 110 to filter the switching frequency of the pulse width modulated signal driving the audio amplifier 102 . Low pass filter 101 may be used, for example, to filter radiated emissions to comply with various government board requirements. Alternatively, the low-pass filter 101 may be omitted.

FIG. 2 shows a system 120 according to another embodiment of the invention. System 120 is similar to system 100 shown in FIG. 1 , except that the transformer coupling RF amplifier 104 to the rest of the circuit is implemented using an electromagnetic transformer or balun 122 instead of DC isolation transformer 110 . In some embodiments, using electromagnetic type transformers instead of magnetic transformers can result in more compact circuits, especially for RF signals with larger bandwidths. In some embodiments, transformer 122 may be implemented using two transformers in a Guanella topology as shown. Alternatively, other electromagnetic transformer types or topologies may be such as, but not limited to, the Ruthroff topology that may be used. In some embodiments, electromagnetic transformer 122 may be implemented using twisted pair cables on magnetic cores, or parallel wiring.

Because transformer 122 does not provide DC isolation between RF amplifier 104 and speaker coil 106 , DC coupling capacitors 132 and 128 are placed in series with transformer 122 to provide DC isolation for RF amplifier 104 . The center tap of transformer 122 may be connected to ground through capacitor 130 . In an alternate embodiment of the invention, the center tap of transformer 122 may be left open and capacitor 130 may be omitted.

The embodiment system 200 shown in FIG. 3 may be used when the audio amplifier 102 and the RF amplifier 104 are configured to transmit at separate times and not simultaneously. In system 200 , RF amplifier 104 is coupled in parallel to speaker coil 106 through AC coupling capacitors 218 and 220 . The audio amplifier 102 is coupled to the speaker coil 106 through a parallel resonant circuit 201 having an inductor 202 and a capacitor 204 and a parallel resonant circuit 203 having an inductor 206 and a capacitor 208 . Capacitors 204 and 208 may be due to parasitic capacitance of transistors 210 and 212, due to additional capacitance coupled between the drains and sources of transistors 210 and 212, or a combination thereof. In an embodiment, the parallel resonant frequency of parallel resonant circuits 201 and 203 corresponds to the RF output frequency generated by RF amplifier 104 . For example, if the RF transmission frequency generated by amplifier 104 is 13 MHz, then the first and second parallel resonant circuits may be configured to have a parallel resonant frequency at approximately 13 MHz. The parallel resonance of parallel resonant circuits 201 and 203 presents a high impedance to RF amplifier 104 at the RF transmit frequency. It should be appreciated that system 200 may be used when audio amplifier 102 is configured to provide a differential output signal. However, in single-ended embodiments, parallel resonant circuit 203 may be omitted.

In an embodiment, the parallel resonant circuit 201 is bypassed by transistor 210 and the parallel resonant circuit 203 is bypassed by transistor 212 . Transistors 210 and 212 may be activated by signal S when audio amplifier 102 is activated and RF amplifier 104 is deactivated. When transistors 210 and 212 are activated, parallel resonant circuits 201 and 203 are bypassed with low impedance, allowing efficient operation of audio amplifier 102 driving speaker coil 106 . On the other hand, when transistors 210 and 212 are disabled, parallel resonant circuits 201 and 203 present a high impedance to RF amplifier 104 at RF frequencies.

In an embodiment, when tuned to the RF transmit frequency in parallel resonance, the off-state capacitances of transistors 210 and 212 are compensated by inductors 202 and 206, respectively. Therefore, a large RF impedance is presented to the speaker coil 106 when the switching transistors 210 and 212 are in the off state. Furthermore, the effect of the large gate capacitance of transistors 210 and 212 can be reduced by adding high ohmic resistors 214 and 216 in series with the gates of transistors 210 and 212, respectively. Resistors 214 and 216 reduce the effect of gate-source and gate-drain capacitance seen on the sources and drains of transistors 210 and 212 . Depending on the type of switching transistor used, bulk control pin VBULK may be coupled to the most positive voltage generating output or the most negative voltage generating output of amplifier 102 to avoid clamping of the audio signal. Because transistors 210 and 212 present a low series impedance (eg, in the mΩ range) to audio amplifier 102 at audio frequencies, inductors 202 and 206 may be implemented using small, high-ohmic inductors. In alternative embodiments of the invention, transistors 210 and 212 may be omitted.

FIG. 4 shows an embodiment system 230 in which the resonant frequency of parallel resonant circuit 201 can be tuned using switching transistors 250 and 252 , and capacitors 232 , 234 , 236 and 238 . Similarly, the parallel resonant frequency of parallel resonant circuit 203 can be tuned using switching transistors 254 , 256 , capacitors 240 , 242 , 244 and 246 . Switching transistors 252 and 256 are driven by signal S1 through resistors 262 and 264 , and switching transistors 250 and 254 are driven by signal S2 through resistors 258 and 260 . Resistors 258, 260, 262, and 264 reduce the effect of the gate capacitance of transistors 250, 252, 254, and 256 on their respective outputs.

The frequency of parallel resonant circuit 201 can be lowered by activating switch 252 , which places capacitors 232 and 238 combined in series in parallel with parallel resonant circuit 201 . This parallel resonant frequency can be further lowered by activating switch 250 , which places series-coupled capacitors 204 and 206 in parallel with parallel resonant circuit 201 . The effect of turning on switches 254 and 256 has a similar effect on the parallel resonant frequency of parallel resonant circuit 203 . Similarly, when the switches 250, 252, 254 and 256 are open, the parallel resonant frequency of the parallel resonant circuits 201 and 203 can be increased. Although only two switching transistors are shown for each parallel resonant circuit 201 and 203 , any number of switches coupled in series with capacitors may be used together to adjust the parallel resonant frequency of parallel resonant circuits 201 and 203 . Therefore, precise control over the parallel resonant frequency of parallel resonant circuits 201 and 203 can be achieved by proper selection of switches and capacitors.

In some embodiments of the invention, switches 250, 252, 254, and 256, and capacitors 232, 234, 236, 238, 240, 242, 244, and 246 may be implemented on a single integrated circuit. Transistors 250, 252, 254, and 256 can be implemented using transistors with low capacitance and high linearity.

System 230 may further be used to dynamically tune the center frequencies of parallel resonant circuits 201 and 203 . For example, the impedance of the speaker coil 106 may change when the system 230 comes within close range of conductive objects such as metal objects, water, and living organisms. High performance can be maintained by tuning the parallel resonance of parallel resonant circuits 201 and 203 in response to environmental changes. In some embodiments, the impedance of the speaker coil 106 is sensed using a second coil close to the speaker coil 106 in transmit mode, or the capacitance of the parallel resonant circuit is tuned for optimal signal-to-noise ratio in receive mode. Tuning may also be used to accommodate systems where the RF amplifier 104 is configured to transmit at different RF transmit frequencies.

In some embodiments, additional bypass switch transistors may be added to bypass parallel resonant circuits 201 and 203 in the same manner as switches 210 and 212 bypassed parallel resonant circuits in system 200 of FIG. 3 . Thus, the bulk node is biased at a voltage greater than the highest output voltage produced by the amplifier 102 or at a voltage less than the most negative voltage produced by the amplifier 102 . In embodiments that do not include these switches, since transistors 250, 252, 254, and 256 are not DC coupled to switching transistors, signal excursions at the output of amplifier 102 that exceed body bias can be tolerated.

Figure 5 shows a system 270 according to another embodiment. System 270 is similar to system 230 of FIG. 4 with the addition of an additional tuning network coupled to the output of audio amplifier 102 . These additional tuning networks can be used to further fine tune the system. Alternatively, an additional tuning network may also be coupled to speaker coil 106 .

Switching transistors 272, 274, 276, and 280 may be coupled to the output of audio amplifier 102 via capacitors 290, 292, 296, and 294, respectively. Series resistors 282 , 284 , 286 , and 288 are coupled in series with the gates of switching transistors 272 , 274 , 276 , and 280 , respectively, to reduce the effect of gate capacitance on the capacitance seen at the output of audio amplifier 102 . It should be appreciated that more switched capacitor units may be coupled in parallel with additional tuning networks to provide further precise tuning of the capacitance seen by speaker coil 106 and/or the series resonant frequency.

In some embodiments, the control terminals S1, S2, S3 and S4 may be controlled remotely based on system feedback or signal levels received at the remote terminal. In other embodiments, an oscillator (not shown) may be used to test the system to determine the state of control terminals S1 , S2 , S3 and S4 .

Switching transistors 250 , 252 , 254 , 256 , 272 , 274 , 276 , and 208 may be implemented using MOS transistors or transistors of another transistor type, such as integrated gate bipolar transistors (IGBTs). In some embodiments, commercial switching transistor components may be used, such as a PGS22 switch. Embodiment systems may be implemented, for example, on an integrated circuit, in one or more integrated circuits mounted in a hybrid package, or on layered circuits using multiple components. In some embodiments, the speaker coil 106 is coupled to the remaining circuit components through a speaker port. The speaker port may include terminals configured to accept speaker coil 106 .

According to an embodiment, a system has an audio amplifier configured to be coupled to a speaker coil port through a parallel resonant circuit, and an RF amplifier configured to transmit an RF signal at a first RF transmit frequency. The speaker coil port is configured to couple to a speaker coil, and the parallel resonant circuit has a resonant frequency about a first radio frequency (RF) transmit frequency. In some cases, the system may further include a speaker coil.

In an embodiment, the parallel resonant circuit includes a first transformer coupled in series with the audio amplifier and the speaker coil port such that the RF amplifier is coupled to the speaker coil port through the first transformer. The first transformer may include a first winding coupled between the audio amplifier and the speaker coil port, and a second winding coupled to the RF amplifier. In further embodiments, the first transformer includes a balun having a first port coupled between the audio amplifier and the speaker coil port, and a second port coupled to the RF amplifier. The balun can be realized using, for example, a Guanella balun.

In some embodiments, an RF amplifier is coupled in parallel with the speaker coil port. In one example, the RF amplifier is coupled to the speaker coil port through a coupling capacitor. The system may further include a bypass switch coupled in parallel with the parallel resonant circuit such that when the audio amplifier is active the bypass switch is configured to be closed and when the audio amplifier is inactive the bypass switch is configured to be open. In this case, the parallel resonant circuit may include an inductor coupled in series with the bypass switch, and the parasitic capacitance of the bypass switch may provide at least a portion of the capacitance of the parallel resonant circuit. MOS transistors or other types of transistors can be used to implement the bypass switch. In some embodiments, a resistor is coupled in series with the gate of the MOS transistor. In another embodiment, the resonant frequency of the parallel resonant circuit is tunable.

According to another embodiment, a circuit includes an audio amplifier configured to be coupled to a speaker coil port through an output port of a transformer coupled in series with the audio amplifier, and an RF amplifier configured to transmit RF signal at the input port. The speaker coil port may be configured to couple to a speaker coil. In some embodiments, the circuit may also include a speaker coil.

In an embodiment, the input port of the transformer includes a first winding and the output port of the transformer includes a second winding. Furthermore, the capacitor may be coupled in parallel with the second winding such that the second winding and the capacitor form a parallel resonant network having a center frequency. The RF amplifier may be configured to transmit at an RF transmit frequency at about a center frequency.

According to another embodiment, a circuit includes an RF amplifier coupled in parallel with a speaker coil port configured to be coupled to a speaker coil. An adjustable parallel resonant circuit having a first port is coupled to the speaker coil port, and an audio amplifier is coupled to a second port of the adjustable parallel resonant circuit. In some circuits, speaker coils are also included. A bypass switch may further be coupled across the parallel resonant circuit such that the bypass switch is configured to be closed when the audio amplifier is active and is configured to be open when the RF amplifier is active.

In an embodiment, the adjustable parallel resonant circuit includes a first parallel resonant circuit coupled between a first terminal of the audio amplifier and a first terminal of the speaker coil port, and a first terminal coupled between a second terminal of the audio amplifier and the speaker coil port. A second parallel resonant circuit between the second terminals. In some cases, an adjustable parallel resonant circuit includes an adjustable capacitor and a fixed inductor. For example, an adjustable capacitor may be implemented using at least one switchable capacitor coupled to a fixed inductor. This switchable capacitor may be a capacitor coupled in series with the MOS switch.

According to another embodiment, a method of operating a speaker coil coupled to an audio amplifier and in parallel with an RF amplifier through an adjustable parallel resonant circuit includes operating the speaker coil in an audio mode and operating the speaker coil in an RF mode. The audio mode includes activating the audio amplifier and deactivating the RF amplifier. Operating the speaker coil in the audio mode may further include bypassing the adjustable parallel resonant circuit. In another aspect, the RF mode includes transmitting an RF signal using an RF amplifier such that the RF signal has a frequency close to a resonant frequency of the adjustable parallel resonant circuit.

In an embodiment, the method may further include adjusting a capacitance of an inductor coupled to the adjustable parallel resonant circuit. Adjusting the capacitance may include activating a switching network coupled to at least one terminal of an inductor, the switching network including a switching transistor coupled in series with at least one capacitor.

Advantages of embodiment systems include the ability to couple audio power amplifiers and radio frequency (RF) power amplifiers to electromagnetic speakers while maintaining high power efficiency of the system. Another advantage includes the ability to provide good signal isolation for audio power amplifiers and RF amplifiers while also providing low insertion loss for both audio and RF signals.

While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims cover any such modifications or embodiments.

Claims (28)

1. A system for transmitting radio frequency RF signals, comprising: an audio amplifier configured to be coupled to a speaker coil port through a parallel resonant circuit having a resonant frequency about the first RF transmit frequency, wherein the speaker coil port is configured to be coupled to the speaker coil; and an RF amplifier configured to transmit the RF signal at the first RF transmit frequency. 2. The system of claim 1, further comprising a speaker coil. 3. The system of claim 1, wherein: the parallel resonant circuit includes a first transformer coupled in series with the audio amplifier and speaker coil ports; and The RF amplifier is coupled to the speaker coil port through the first transformer. 4. The system of claim 3, wherein said first transformer comprises: a first winding coupled between the audio amplifier and the speaker coil port; and coupled to the second winding of the RF amplifier. 5. The system of claim 3, wherein said first transformer comprises a balun having a second balun coupled between said audio amplifier and said speaker coil port. One port and a second port coupled to an RF amplifier. 6. The system of claim 5, wherein the balun comprises a Guanella balun. 7. The system of claim 1, wherein the RF amplifier is coupled in parallel with the speaker coil port. 8. The system of claim 7, wherein the RF amplifier is coupled to the speaker coil port through a coupling capacitor. 9. The system of claim 7, further comprising a bypass switch coupled in parallel with the parallel resonant circuit, the bypass switch being configured to be closed when the audio amplifier is active, and to be closed when the audio amplifier is inactive , the bypass switch is configured to be open. 10. The system of claim 9, wherein: the parallel resonant circuit includes an inductor coupled in series with the bypass switch; and The parasitic capacitance of the bypass switch provides at least a portion of the capacitance of the parallel resonant circuit. 11. The system of claim 9, wherein the bypass switch comprises a MOS transistor. 12. The system of claim 11, further comprising a resistor coupled in series with the gate of the MOS transistor. 13. The system of claim 7, wherein the resonant frequency of the parallel resonant circuit is tunable. 14. A circuit comprising: an audio amplifier configured to be coupled to a speaker coil port through an output port of a transformer coupled in series with the audio amplifier, wherein the speaker coil port is configured to be coupled to a speaker coil; and An RF amplifier configured to transmit an RF signal coupled to the input port of the transformer. 15. The circuit of claim 14, further comprising said speaker coil. 16. The circuit of claim 14, wherein: the input port of the transformer includes a first winding; and The output port of the transformer includes a second winding. 17. The circuit of claim 16, further comprising a capacitor coupled in parallel with the second winding, wherein the second winding and the capacitor form a parallel resonant network having a center frequency; and The RF amplifier is configured to transmit at an RF transmit frequency at about a center frequency. 18. A circuit comprising: an RF amplifier coupled in parallel with a speaker coil port configured to couple to the speaker coil; an adjustable parallel resonant circuit having a first port coupled to a speaker coil port; An audio amplifier coupled to the second port of the adjustable parallel resonant circuit. 19. The circuit of claim 18, further comprising said speaker coil. 20. The circuit of claim 18 , further comprising a bypass switch coupled across said parallel resonant circuit, said bypass switch being configured to be closed when said audio amplifier is active, and when said The bypass switch is configured to be open when the RF amplifier is active. 21. The circuit of claim 18, wherein said adjustable parallel resonant circuit comprises: a first parallel resonant circuit coupled between a first terminal of the audio amplifier and a first terminal of the speaker coil port; and A second parallel resonant circuit is coupled between the second terminal of the audio amplifier and the second terminal of the speaker coil port. 22. The circuit of claim 18, wherein the adjustable parallel resonant circuit includes an adjustable capacitor and a fixed inductor. 23. The circuit of claim 22, wherein the adjustable capacitor comprises at least one switchable capacitor coupled to a fixed inductor. 24. The circuit of claim 23, wherein the switchable capacitor comprises a capacitor coupled in series with a MOS switch. 25. A method of operating a speaker coil coupled to an audio amplifier and in parallel with an RF amplifier via an adjustable parallel resonant circuit, the method comprising: operating the speaker coil in an audio mode includes activating an audio amplifier and deactivating the RF amplifier; and Operating the speaker coil in RF mode includes transmitting an RF signal using the RF amplifier, the RF signal having a frequency close to the resonant frequency of the adjustable parallel resonant circuit. 26. The method of claim 25, wherein operating the speaker coil in the audio mode includes bypassing the adjustable parallel resonant circuit. 27. The method of claim 25, further comprising adjusting the capacitance of an inductor coupled to the adjustable parallel resonant circuit. 28. The method of claim 27, further wherein adjusting the capacitance comprises activating a switching network coupled to at least one terminal of an inductor, the switching network comprising a switching transistor coupled in series with at least one capacitor.